Allogeneic hematopoietic stem cell transplantation (HSCT) is a potentially curative treatment for a variety of hematological malignancies. The major complications of HSCT include graft failure, opportunistic infections, leukemic relapse and graft-versus-host disease (GVHD). The incidence of the first three complications is diminished by the presence of mature donor T cells in the transplant inoculum and the breadth and speed of T cell reconstitution. Unfortunately, the transplantation of mature alloreactive donor T cells also directly induces the latter complication of acute GVHD. The key to the future success of allogeneic HSCT as a cancer therapy then lies in the ability to enhance the beneficial effects of the donor T cells in mediating graft-versus-leukemia effects (GVL) while minimizing their capacity to cause GVHD. The general aim of this current proposal is the investigation of the immunobiology of lethal GVHD and GVL effects by concentrating on how the donor anti-host minor histocompatibility antigen (miHA) T cell repertoire develops and the broad definition of the TCR specificities involved in disease pathogenesis and GVL responses. To accomplish this, the B 10.BR-greater than CBA and the C5 7BL/6 (B6)-greater than CXBE miHA-mismatched strain combination transplantation models will be utilized. The GVHD generated in these models are both mediated by CD8 +T cells, but differ in their dependence upon CD4 T helper activity, the kinetics, and dose-dependent pathogenesis of the response. The T cell repertoires involved in GVHD and GVL responses will be analyzed by TCR Vbeta spectratyping. To perform the latter study, B10.BR and B6 mice will be challenged with myeloid leukemia cells (MMC6 and MME4) that have been derived from either CBA or CXBE mice, respectively. The spectratype analysis will be used to guide manipulation of donor T cell inocula used along with HSCT, based on the strength of individual T cell Vbeta family responses, in an effort to boost GVL activity while diminishing GVHD. These pre-clinical murine studies should establish the foundation for the human studies in which TCR VJ3 spectratype analysis will be used to examine the predictive value of in vitro limiting dilution culture T cell responses, generated between donor and host patient pairs, for the in vivo GVHD response. These in vitro analyses will be compared with TCR Vbeta spectratyping of peripheral blood lymphocyte (PBL) samples obtained from the same patients post-HSCT. Matching Vbeta spectratypes between the in vitro and in vivo analysis would suggest that in vitro analysis would be useful for predicting which T cell responses are involved in GVHD and could be used to direct GVHD prophylaxis or possibly guide the manipulation of the transplant inoculum.